Ethylene Polymerization with a Hafnocene Dichloride Catalyst Using Trioctyl Aluminum and Borate: Polymerization Kinetics and Polymer Characterization

Abstract

The polymerization of ethylene is investigated in a semibatch solution reactor using bis(n‐propylcyclopentadienyl)hafnium dichloride catalyst and tetrakis(pentafluorophenyl) borate dimethylanilinium salt ([B(C6F5)4]−[Me2NHPh]+) as the catalytic system. Trioctylaluminum (TOA) is used as impurity scavenger and alkylating agent. Ethylene pressure, polymerization temperature, TOA, borate, and catalyst concentrations are changed to investigate ethylene polymerization kinetics with this catalyst system. A 23 central composite design, augmented with extra runs to further explore the effect of some factors, is used as the statistical basis for the polymerization study. Ethylene propagation follows first‐order kinetics. Chain transfer to monomer, β‐hydride elimination, and transfer to TOA are the main chain transfer reactions. In addition to alkylating the catalyst precursor, TOA also deactivates the catalyst. The mode of reactor addition for catalyst, borate, and TOA has also been studied. When TOA and borate are added sequentially to the reactor, followed by the catalyst, the polymerization activity is lower than when the catalyst and borate are added simultaneously, suggesting that complexation with borate avoids deactivation reactions with TOA.

Atypical Multiple Site Behavior of Hafnocene Catalysts in Ethylene/1-Hexene Copolymerization Using Trioctylaluminum and Borate

“…In previous publications from our group, , the effects of activating rac- dimethylsilyl­bis­(indenyl)hafnium dimethyl with MAO or tetrakis­(penta­fluorophenyl) borate dimethyl­anilinium salt were compared using statistical techniques to estimate the main polymerization kinetic parameters for these systems. This line of research was extended in a subsequent publication to study the kinetics of ethylene polymerization using the bis­( n -propylcyclo­pentadienyl)­hafnium dichloride/borate/TOA system . The present investigation continues our efforts to elucidate these intriguing systems by by providing additional details on the copolymerization of ethylene and 1-hexene using bis­( n -propylcyclo­pentadienyl)­hafnium dichloride (HfCl 2 ) and bis­( n -propylcyclo­pentadienyl)­hafnium dimethyl (HfMe 2 ) activated with tetrakis­(pentafluoro­phenyl) borate dimethyl­anilinium salt (B), in the presence of trioctylaluminum (TOA).…”

“…When catalyst deactivation and ethylene propagation rates follow first-order kinetics and also assuming that the site activation is instantaneous, the following equation describes ethylene uptake curves , where F M,in , is the molar flow rate of ethylene to the reactor, [M] is ethylene concentration in the liquid phase, [C i ] 0 is the initial catalyst concentration, and V R is the volume of the reaction medium. Equation can be linearized through the simple transformation …”

“…Consequently, the polymerization rate slows down, and the polymer yield drops as more 1-hexene is added to the reactor. When catalyst deactivation and ethylene propagation rates follow first-order kinetics and also assuming that the site activation is instantaneous, the following equation describes ethylene uptake curves 35,36…”

“…This line of research was extended in a subsequent publication to study the kinetics of ethylene polymerization using the bis(npropylcyclopentadienyl)hafnium dichloride/borate/TOA system. 35 The present investigation continues our efforts to elucidate these intriguing systems by by providing additional details on the copolymerization of ethylene and 1-hexene using bis(n-propylcyclopentadienyl)hafnium dichloride (HfCl 2 ) and bis(n-propylcyclopentadienyl)hafnium dimethyl (HfMe 2 ) activated with tetrakis(pentafluorophenyl) borate dimethylanilinium salt (B), in the presence of trioctylaluminum (TOA). In this paper, we show for the first time how CCD bimodality can be tuned by varying the ratio of catalyst/TOA and catalyst/B for either one of the molecular catalysts studied in this investigation.…”

Solution Structure and Dynamics of Hf–Al and Hf–Zn Heterobimetallic Adducts Mimicking Relevant Intermediates in Chain Transfer Reactions

“…We recently showed that a typical ion pair active species [(Cp Pr ) 2 HfMe]­[B­(C 6 F 5 ) 4 ] ( 3 Pr , Cp Pr = n -propyl-cyclopentadienyl ligand) undergoes selective activation of a propyl chain of a substituted Cp ligand, affording the thermally stable cyclometalated species [Cp Pr Cp CH2CH2CH2 Hf]­[B­(C 6 F 5 ) 4 ] ( 4 Pr ). , This in situ modification was proposed to contribute to multisite behavior observed for this and related catalysts. − Due to the structural similarities of 4 Pr with an active catalytic species bearing a nascent polymeryl chain in which the last inserted unit is an ethylene molecule (Scheme ), we further explored the reactivity of 4 Pr with several CTAs. We succeeded to structurally characterize, through NMR spectroscopy and DFT calculations, a series of Hf–Al and Hf–Zn heterobimetallic adducts featuring inequivalent bridging alkyls and showed that these species are stabilized by multiple α-agostic interactions .…”